Hard disk drives store information on magnetic disks. Typically, the information is stored on concentric tracks of the disk that are divided into servo sectors and data sectors. Information is written to or read from the disk by a transducer or head, mounted on an actuator arm that positions the transducer over the disk in a predetermined location. Accordingly, the movement of the actuator arm allows the transducer to access the different tracks of the disk. The disk is rotated by a spindle motor at a high speed, allowing the transducer to access different sectors within each track of the disk.
A voice coil motor in combination with a servo control system is usually employed to position the actuator arm. The servo control system performs the function of seek control and track following. The seek function is initiated when a command is issued to read data from or write data to a target track on the disk. Once the transducer has been positioned sufficiently close to the target track by the seek function of the servo control system, the track following function of the control system centers and maintains the transducer on the target track until the desired data transfer is completed.
Typically, the transducer will oscillate about the centerline of the target track for a period of time following the transition of the servo control system from the seek mode to the track following mode. These off-track displacements, or post-seek oscillations (PSO), are due, at least in part, to mechanical vibrations generated by the components of the disk drive during the seek and/or tracking operation. In addition, while in the track following mode, adjustments to the position of the transducer with respect to the centerline of the target track are often required due to these same or similar mechanical vibrations. Such adjustments are required to correct drift in the position of the transducer relative to the target track. The precise control of the position of the transducer relative to a target track has become increasingly important as track densities (or tracks per radial inch—TPI) in disk drives have increased. More specifically, the number of tracks included on a disk, i.e., the greater the TPI, translates to higher data storage capability. However, the increased number of tracks means that there is a more stringent requirement that the transducer stay on track for both reading and writing purposes as the separation distance between adjacent tracks decreases. A measure of how far the transducer is off target is termed “Track Misregistration” (TMR). It can be measured in distance (e.g., microns) or as a percentage of track pitch. TMR is also referred to as off track or track following errors.
The actuator assembly generally includes one or more actuator arms interconnected to a part, commonly known as an E-block in the art, having an aperture for receiving a shaft and associated bearing that allows the actuator assembly to freely rotate about the axis of the shaft. Each actuator arm includes a load beam supplying a slider which incorporates the previously described transducer. Components of the actuator assembly generate vibrational loads that impair the ability of the actuator assembly to position and maintain the transducer over a desired track. The actuator assembly also includes a yoke and voice coil that contribute to the vibrational loads. To account for vibrational loads, during the design phase, the amount of vibration from the assembled components may be assigned a budget that must not exceed a predetermined level of generated vibration, thus minimizing TMR and post seek oscillation errors. These budgets are based upon vibrations originating from a number of sources and take on various forms including, but not limited to, electrical noise torque, whirl, arm mode, drum mode, ball bearing tones, high frequency turbulence, disk vibration, aerodynamic torque, and external vibration or seek settle. More specifically, the vibrational loads are generated by the different modes of vibrational motion generated by the components of the actuator assembly. Minute vibrational loads that emanate from the aerodynamic loading of the disk and/or actuator moving through the air inside the disk drive housing may also affect TMR and post seek oscillations. Thus, it is important for designers of disk drives to reduce the individual sources of vibrational loads and/or alter resonance frequencies of disk drive components that influence positioning of the transducer to produce a disk drive that allows the servo control system to better compensate for post seek oscillations and TMR.
In addition to the post seek oscillations and TMR generated by the components of the actuator assembly, post seek oscillations are also caused by the acceleration and deceleration of the actuator assembly as the actuator arms) moves from a current track to a target or intended track. As the actuator moves, resonance frequencies may be exited. The primary or system resonant mode is the most significant in that it limits the ability of the servo control system to compensate for the PSO and TMR and is evidenced by in-plane butterfly-like displacements of the actuator head relative to the voice coil. The system mode frequency is generally on the order of about 5-10 Khz.
The negative effects of post seek oscillations and TMR are most easily described by a brief discussion of track pitch. The distance between two concentric tracks of a disk is known as track pitch, which decreases as TPI increases. For example, a disk with 100,000 TPI has generally a track pitch budget of 0.25 microns (approximately 10 millionths of an inch), wherein a disk with a 150,000 TPI has a track pitch of about 0.17 microns (approximately 7 millionths of an inch). As described above, each vibrating component of a disk drive has a budget that contributes to the maximum allowable TMR that are correctable by the servo control system. That is, vibrational induced oscillations of the transducer must be maintained at or below a level where the servo controller can effectively counteract the movement and control the position of the transducer. This level is predetermined in the design of a disk drive. Returning now to the above example in which TPI is increased from 100,000 to 150,000, and the same servo controller is used in each instance, vibrations generated by the disk drive components increase as a percentage of the total budget. Therefore, it is desirable to implement means of reducing vibrations such as by stiffening the actuator assembly and/or reducing mass or rotating inertia to effectively shift the frequency of the system mode.
It is thus often desirable to stiffen the pivot bearing to alter the natural frequency of the actuator assembly thereby allowing the servo system of the disk drive to be more able to counteract vibration from other components of the disk drive. The E-block of the actuator assembly is usually rotatably interconnected to a plurality of ball bearings and a spacer to a stationary shaft. Although embodiments contemplated herein are based on ball bearings, one skilled in the art will appreciate that other types of bearings, such as roller, needle, etc., may be employed without departing from the scope of the invention. The stationary shaft includes a flange at one end for interconnection to the base plate of a disk drive housing. An upper ball bearing assembly and a lower ball bearing assembly are positioned around the external diameter of the shaft. Each ball bearing assembly is comprised of an inner race, which is interconnected to the stationary shaft, and an outer race, which is interconnected to the E-block. The inner and outer races are separated by a plurality of ball bearings. A shaped spacer separates the upper ball bearing assembly and the lower ball bearing assembly and acts as a labyrinth seal. The outer race of the upper ball bearing assembly and the lower ball bearing assembly are bonded to the E-block. The spacer functions to separate the ball bearing assemblies and to form a labyrinth seal with the shaft/stiffener. This type of assembly is generally known as a “sleeveless cartridge”, other pivot assemblies of the prior art include a cylindrical sleeve positioned exterior to the ball bearings and the spacer and bonded to the E-block. As one skilled in the art will appreciate, a stiffener can be employed along with an actuator bearing cartridge assembly with a sleeve that does not necessarily reduce the rotating mass of the E-block but still improves the system mode frequency.
Currently, interconnection of the E-block to the pivot bearing includes bonding the outer diameters of the upper and lower ball bearing assemblies to the inner diameter of an aperture integrated into the E-block. In order to interconnect the components, a tolerance ring may be employed, which is generally a corrugated metal strip that acts as an interface between two mechanical objects, to secure the ball bearing assemblies to the E-block by interference fit. The typical assembly process is separated into two separate processes/operations: 1) the assembly of the bearing cartridge and 2) the assembly of the completed bearing cartridge into the E-block. Generally, the bearing cartridge sub-assembly requires an adhesive that is cured at room temperature to substantially avoid thermal stresses. The cartridge is then vacuum baked to eliminate high volatility components from the adhesives. The hearing cartridge is bonded to the E-block at a high temperature that is conducive to eliminating these volatile elements from the adhesive that is not too high wherein thermal expansion of the E-block overstresses the bearing cartridge. An adhesive or bonding material may also be used between the outer bearing races and the E-block.
Heat is also generated by normal disk drive operations that causes thermal expansion of the shaft and the spacer. If the material of the shaft is not the same or substantially similar to the spacer, the inner and outer races of the ball bearing assemblies will move relative to each other. This relative movement may damage the ball bearing assemblies thereby causing track misregistration. More specifically, the thermal expansion rate of stainless steel differs from the thermal expansion rate of aluminum. Generally, aluminum has a linear rate of expansion of 24×10−6 per degree centigrade and stainless steel has a linear rate of expansion on the order of ˜10×10−6 per degree centigrade. Accordingly, where the shaft is made from one of these two materials and the spacer is made from the other, relative movement can occur between the two due to the different rates of thermal expansion. In addition, as the shaft and the spacer are heated and cooled, forces generated by the different rates of expansion and contraction may cause ball bearing debonding from the E-block.